Many atomic nuclei possess a magnetic moment. Nuclear magnetic resonance (NMR) is a phenomenon exhibited by this select group of atomic nuclei (termed "NMR active" nuclei), which results from the interaction of the nuclei with an applied, external magnetic field.
The magnetic properties of a nucleus are conveniently discussed in terms of two quantities: the magnetogyric ratio (denoted by the symbol .gamma.); and the nuclear spin (denoted by the symbol I). When an NMR active nucleus is placed in a magnetic field, its nuclear magnetic energy levels are split into (2I+1) non-degenerate energy levels, and these levels are separated from each other by a characteristic energy that is directly proportional to the strength of the applied magnetic field. This phenomenon is called "Zeeman" splitting and the characteristic energy is equal to .gamma.h H.sub.o /2.pi., where h is Plank's constant and H.sub.o is the strength of the magnetic field. The frequency corresponding to the energy of the Zeeman splitting (.omega..sub.o =.gamma.H.sub.o) is called the "Larmor frequency" or "resonance" frequency. Typical NMR active nuclei include .sup.1 H (protons), .sup.13 C, .sup.19 F, and .sup.31 P nuclei For these four nuclei, the nuclear spin I=1/2, and, accordingly, each nucleus has two nuclear magnetic energy levels.
When a bulk material sample containing NMR active nuclei is placed within a magnetic field, the nuclear spins distribute themselves amongst the nuclear magnetic energy levels in a known manner in accordance with Boltzmann's statics. This distribution results in a population imbalance between the energy levels and a net nuclear magnetization. It is this net nuclear magnetization that is studied by NMR techniques.
At equilibrium, the net nuclear magnetization is aligned with the external magnetic field and is time-independent. A second magnetic field perpendicular to the first magnetic field and rotating at, or near, the Larmor frequency can also be applied to the nuclei and this second field disturbs the equilibrium and induces a coherent motion (a "nutation") of the net nuclear magnetization. Since, at conventional magnetic field strengths, the Larmor frequency of typical NMR active nuclei is in the megahertz range, this second field is called a "radio frequency field" (RF field). The effect of the RF field is to rotate the spin magnetization about the direction of the applied RF field. The time duration of the applied RF field determines the angle through which the spin magnetization nutates and, by convention, an RF pulse of sufficient length to nutate the nuclear magnetization through an angle of 90.degree., or .pi./2 radians, is called a ".pi./2 pulse".
A .pi./2 pulse applied at a frequency near the resonance frequency will rotate a spin magnetization that was aligned along the external magnetic field direction in equilibrium into a plane perpendicular to the external magnetic field. The component of the net magnetization that is transverse to the external magnetic field then precesses about the external magnetic field at the Larmor frequency. This precession can be detected with a resonant coil located with respect to the sample such that the precessing magnetization induces a voltage across the coil. Frequently, the "transmitter" coil employed to apply the RF field to the sample and cause the spin magnetization to nutate and the "receiver" coil employed to detect the resulting precessing magnetization are one and the same coil. This coil is generally part of an NMR probe.
In addition to precessing at the Larmor frequency, the magnetization induced by the applied RF field changes and reverts to the equilibrium condition over time as determined by two relaxation processes: (1) dephasing within the transverse plane ("spin-spin relaxation") with an associated relaxation time, T.sub.2, and (2) a return to the equilibrium population of the nuclear magnetic energy levels ("spin lattice relaxation") with an associated relaxation time, T.sub.1.
When an external magnetic field is applied to a nuclei in a chemical sample, the nuclear magnetic moments of the nuclei each experience a magnetic field that is reduced from the applied field due to a screening effect from the surrounding electron cloud. This screening results in a slight shift of the Larmor frequency for each nucleus (called the "chemical shift" since the size and symmetry of the shielding is dependent on the chemical composition of the sample).
In addition to the applied external magnetic field, each nucleus is also subject to local magnetic fields such as those generated by other nuclear and electron magnetic moments associated with nuclei and electrons located nearby. Interaction between these magnetic moments are called "couplings", and one important example of such couplings is the "dipolar" coupling. In solids, the NMR spectra of spin=1/2 nuclei are often dominated by dipolar couplings, and in particular by dipolar couplings with adjacent protons.
Normally, it is desirable to construct an NMR probe so that the RF field is homogeneous over the entire sample volume, so that an RF pulse generated by the probe uniformly excites all spins of a given nuclear type. However, in some cases, it is useful to intentionally build an NMR probe such that the RF field strength generated by the probe varies across the sample volume. For example, in certain NMR spatial imaging studies, the volume of the material under study may be too large to be entirely enclosed in a single RF coil; alternatively only a small portion of the sample may be of interest. Consequently, in these studies, RF coils are placed on the surface of the material and used to remotely generate RF fields within the body of the material. The variation of the RF field strength produced by the coils can then be used to selectively excite only those nuclei that are of interest. Additionally, in such situations the effective "filling factor" of the receiver coil is increased resulting in an improved signal to-noise ratio. In these situations it is important that the nuclear spins be excited only at the remote site in order to avoid interference signals generated by the spins in intervening locations.
In other studies, it is desirable to use techniques which excite nuclei in a selected plane or slice. For example, prior to recording a two dimensional NMR image, it is normal to selectively excite those nuclei that are located within a thin plane of the sample so that the final NMR image reveals the variations in nuclear spin density within this plane. One approach to generating a variable RF field strength in connection with these latter imaging studies is to use an RF field strength selective pulse sequence. Such an RF field strength selective pulse sequence would also be useful for spatially selective surface coil measurements in solids previously mentioned.
There are also applications in NMR spectroscopy which benefit from RF field strength selective excitation. For instance, although it is desirable to apply the RF excitation pulse so that the RF field is substantially constant over the entire sample, the RF field in a conventional coil is not constant and uniform due to end effects of the coil and due to the discrete wire placement of the coil construction. In experiments which use multiple pulse sequences that function best at a precise, predefined RF field strength, it is desirable to use RF field strength selective excitation in order to selectively excite only those nuclei that are in the correct RF field strength. Background resonances from materials that are near, but outside of the RF coil can also be a problem, and since the background signal originates from areas where the RF field strength is weaker than it is within the coil, it is again desirable to use RF field strength selective excitation in order to selectively excite only those nuclei that are in correct RF field strength.
Traditionally, RF field strength selective sequences have been composed of phase cycled .pi. pulses, n.pi. pulses or composite pulse versions of these latter pulse types. Unfortunately, a single .pi. pulse is not very selective and, therefore, large strings of .pi. pulses must frequently be employed to obtain a reasonable selectivity. A string of n.theta. pulses creates an RF profile with the functional form of (cos.theta.).sup.n. In order to improve the selectivity, other known techniques have been employed including the use of retrograde composite .pi. pulse sequences. Dipolar decoupled composite inversion pulses have also been used in surface coil studies of abundant nuclei in solid samples.
Accordingly, it is an object of the present invention to provide an RF field strength selective excitation method.
It is another object of the present invention to provide an RF field strength selective excitation method that is applicable to both liquid and solid samples.
It is still another object of the present invention to provide an RF field strength selective excitation method that is capable of selectively exciting those nuclear spins that are located in a very narrow range of RF field strengths.
It is yet another object of the present invention to provide an RF field strength selective excitation method that allows for simple variability in the width and mean of the selected field strengths.
It is a further object of the present invention to provide an RF field strength selective excitation method that is compatible with conventional selective RF pulse sequence techniques employing phase cycling, n.pi. pulse sequences, and retrograde composite pulse sequences.
It is still a further object of the present invention to provide an RF field strength selective excitation method that is compatible in most case with conventional dipolar decoupling RF pulse sequences.